Amplifiers have been used in various electronic apparatuses. Amplifiers are commonly known to have a highest efficiency in an output saturation region (i.e., in a nonlinear state).
An amplification apparatus of outphasing type (hereinafter, sometimes referred to as an “outphasing amplifier”) has been proposed as an amplification apparatus that operates its amplifiers in an output saturation region. In other words, an outphasing amplifier is a LINC (Linear Amplification with Nonlinear Components) linear amplifier using nonlinear amplifiers (i.e., saturated amplifiers).
FIG. 1 is a block diagram illustrating an example of a related outphasing amplifier. As illustrated in FIG. 1, the outphasing amplifier includes a signal component separator (SCS), a first nonlinear amplifier (amplifier A) of a “first branch,” and a second nonlinear amplifier (amplifier B) of a “second branch.”
The signal component separator separates an input modulation signal Sin(t) including envelope variations, input to an input terminal into a phase modulation signal pair Sc1(t) and Sc2(t) having a phase difference according to the amplitude of the input modulation signal Sin(t), and outputs the phase modulation signal pair Sc1(t) and Sc2(t). Hereinafter, Sc1(t) may be referred to as a “first branch signal,” and Sc2(t) as a “second branch signal.”
For example, the input modulation signal Sin(t) is a modulation signal with amplitude modulation and phase modulation (angular modulation). The phase modulation signal pair Sc1(t) and Sc2(t) includes constant amplitude phase modulation signals having a constant envelope. Both the input modulation signal Sin(t) and the phase modulation signal pair Sc1(t) and Sc2(t) may be baseband signals or IF signals. The signal component separator generates the phase modulation signal pair Sc1(t) and Sc2(t) as digital signals.
Sc1(t), one of the phase modulation signal pair generated by the signal component separator, is converted from a digital signal into an analog signal by a D/A converter. Sc1(t) is further passed through a filter, so that components corresponding to the frequency band of Sc1(t) are extracted and the other frequency components are suppressed. Similarly, Sc2(t), the other of the phase modulation signal pair, is converted from a digital signal into an analog signal by a D/A converter. Sc2(t) is further passed through a filter so that components corresponding to the frequency band of Sc2(t) are extracted and the other frequency components are suppressed.
An orthogonal modulator orthogonally modulates Sc1(t) passed through the filter by using a high frequency signal (oscillation signal) SL(t) (not illustrated) output from an oscillator, thereby generating and outputting S1(t) which is an RF signal. Similarly, an orthogonal modulator orthogonally modulates Sc2(t) passed through the filter by using the high frequency signal SL(t) output from the oscillator, thereby generating and outputting S2(t) which is an RF signal.
The input modulation signal Sin(t) will be expressed by the following equation (1). Then, the phase modulation signal pair Sc1(t) and Sc2(t) and the high frequency signal pair S1(t) and S2(t) can be expressed by the following equations (2) to (6):Sin(t)=a(t)·cos [θ(t)]  (1)Sc1(t)=amax·cos [θ(t)+ψ(t)]  (2)Sc2(t)=amax·cos [θ(t)−ψ(t)]  (3)S1(t)=amax·cos [2·π·fc·t+θ(t)+ψ(t)]  (4)S2(t)=amax·cos [2·π·fc·t+θ(t)−ψ(t)]  (5)ψ(t)=cos−1 [a(t)/(2·amax)]  (6)
In equations (1) to (6), a(t) is the amplitude modulation component of the input modulation signal Sin(t). θ(t) is the phase modulation component (angular modulation component) of the input modulation signal Sin(t). fc is the frequency of the high frequency component SL(t) output from the oscillator, i.e., the carrier frequency of the high frequency signal pair S1(t) and S2(t). amax is a constant that is set based on a saturation output level of the amplifier pair (i.e., the foregoing first and second nonlinear amplifiers). Such a configuration including the signal component separator, the oscillator, and the orthogonal modulators is used to generate the high frequency signal pair S1(t) and S2(t) that are modulated in phase to produce a phase difference of 2×ψ(t) according to the amplitude of the input modulation signal Sin(t).
As described above, the amplifier pair includes the first nonlinear amplifier and the second nonlinear amplifier which are arranged in parallel with each other. The first nonlinear amplifier and the second nonlinear amplifier have generally the same gain and phase characteristics. The first nonlinear amplifier amplifies S1(t), one of the high frequency signal pair, output from the orthogonal amplifier. The second nonlinear amplifier amplifies S2(t), the other of the high frequency signal pair, output from the orthogonal amplifier.
A combiner combines a high frequency signal pair G×S1(t) and G×S2(t) amplified by the amplifier pair (G is the gain of the amplifiers) and outputs the combined signal as an output high frequency signal Sout(t) from an output terminal. Assuming that the pass phase of the high frequency signal pair S1(t) and S2(t) is (1), the output high frequency signal Sout(t) can be expressed by the following equation (7):Sout(t)=G·amax·cos [2·π·fc·t+θ(t)+ψ(t)+φ]+G·amax·cos [2·π·fc·t+θ(t)−ψ(t)+φ]=2·G·amax·cos [2·π·fc·t+θ(t)+φ]·cos [ψ(t)]=G·a(t)·cos [2·π·fc·t+θ(t)+φ]  (7)
As expressed in equation (7), the outphasing amplifier provides the output high frequency signal Sout(t) obtained by amplifying the input modulation signal Sin(t) by the gain G, with highly efficient linear amplification.
Meanwhile, the outphasing amplifier causes distortion due to characteristic variations and the like of the two nonlinear amplifiers. To suppress such a nonlinear distortion and reduce an adjacent channel leakage ratio (ACLR), some wireless transmission apparatuses equipped with an outphasing amplifier may include a distortion compensation device for compensating the nonlinear distortion.
For example, some wireless transmission apparatuses including a related outphasing amplifier may include a distortion compensation device that calculates distortion characteristics due to a characteristic difference between the two nonlinear amplifiers (hereinafter, may be referred to as “branch distortion characteristics”) and a distortion characteristic of the entire outphasing amplifier (hereinafter, may referred to as “overall distortion characteristic”) and further calculates inverse characteristics of the respective distortion characteristics (i.e., “inverse branch characteristics” and “inverse overall characteristic”). The distortion compensation device then multiplies a transmission baseband signal by the inverse overall characteristic in an input stage of the outphasing amplifier. The signal component separator of the outphasing amplifier decomposes the input signal into the two branch signals having a phase difference according to the amplitude of the input signal. The distortion compensation device multiplies the branch signals by the respective inverse branch characteristics. In such a manner, the overall distortion characteristic of the outphasing amplifier and the balance between the branches can be compensated. In other words, the nonlinear distortion of the outphasing amplifier can be compensated.
[Patent Document 1] Japanese Laid-open Patent Publication No. 2014-011653
However, to compensate the overall distortion characteristic of the outphasing amplifier and the balance between the branches, the foregoing related distortion compensation device calculates the overall distortion characteristic and the branch distortion characteristics before calculating the inverse characteristics thereof as described above. This entails a high calculation load.